The Effects of CsBr Concentration on the Inorganic Cesium Lead Bromide Perovskite Film Properties and the Performances of Carbon-Based HTM-Free Perovskite Solar Cells

Inorganic cesium lead bromide (ICLB) perovskite films were prepared onto an FTO conductive substrate by a two-step spin-dipping method. PbBr₂ films were first coated onto the FTO substrate, and then they were immersed into CsBr solutions at various concentrations: 0.04, 0.06, 0.08, 0.10, and 0.12 M, forming the ICLB perovskite films. The surface morphology of the perovskite films prepared from the CsBr concentrations under 0.08 M had a uniform crystalline surface, but the CsBr concentrations above 0.08 M gave the film a non-uniform structure. XRD spectra of all ICLB films compose of mixed phases of monoclinic-CsPbBr₃ and tetragonal-CsPb₂Br₅. The direct optical bandgap of 2.3 eV corresponded to the CsPbBr₃ phase, and the indirect optical bandgap of 2.87-3.10 eV corresponded to the CsPb₂Br₅ phase. Carbon-based hole-transport-material (HTM) free CsPb₂Br₅ - CsPbBr₃ perovskite solar cells were assembled, and the CsPb₂Br₅ - CsPbBr₃ perovskite solar cells prepared from 0.08 M CsBr concentration delivered the highest efficiency of 2.6%. This was because the 0.08 M-perovskite film had good uniformity, low pinhole defect, and low PbBr₂ impurities. Good cell stability, with an efficiency reduction of 10.0% of the initial value after 816 h under ambient environment, was achieved from the 0.08 M CsBr concentration cells.

Keywords: CsBr; inorganic cesium lead bromide perovskite; solar cells; hole transport material free

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